This invention relates to the assembly or packaging of semiconductive optoelectronic devices and, in particular, to an improved method for accurately assembling devices for use in optical fiber communication systems.
As known in the art, communication by way of optical fiber has many advantages over communication by way of other media. An obstacle to the increased use of optical fiber media is the cost of the components or, more directly, the distance over which one can communicate directly, i.e. without a repeater or other intervening means. In part, this distance is determined by the characteristics of the optical fiber. In part, the distance is determined by the power (flux density) of the light emitted and the sensitivity of the detector.
There are two ways, basically, to increase the flux density in the fiber. One is to use a higher power emitter. The other is to increase the efficiency of the coupling of light from the emitter to the fiber. On the receiving end, one has the corresponding choices of increasing the performance, and cost, of the light detector or increasing the efficiency of the optical coupling from the fiber to the detector.
In the prior art, the use of glass beads as lenses increased the coupling efficiency into and out of the semiconductor die. In Ser. No. 272,822, filed June 12, 1981 (now abandoned) and assigned to the assignee of the present invention, efficiency and light control are described as further improved by controlling the spacing between the semiconductor and the lens.
A complete semiconductor device includes a die, which may comprise an emitter, a detector, or both; a header for mechanical support, electrical connections, and heat spreading; and a cap having a window through which the light passes. There is also usually a thermally conductive, electrically insulating substrate between the semiconductor die and the header.
Despite the advances in coupling efficiency described above, much light is lost between the lens and the fiber due to misalignment within the package. This is due to the cumulative error in assembling the various parts. While the lens may be accurately placed radially with respect to the photoactive area on the semiconductor die, there is a tolerance in locating the die on the substrate, the die and substrate on the header, and the cap on the header. Since the optical fiber in a connector is located with respect to the outside surface of the cap, the cumulative error can be devastating. More specifically, yields go down and costs go up.
At present, two approaches have been used to solve the problem. The first is to specify tight tolerances for all parts and assembly operations. This technique produces devices with acceptable characteristics but at high cost. The alternative is to design devices to accommodate radial misaligment.
The art at present typically uses fiber having a core diameter of 200 micron (8 mil). The package is designed with a projected spot size of 300 micron (12 mil), i.e. the size of the image of the emitter as seen at the window at the top of the cap. Despite using precision parts, one can obtain a worst-case cumulative error of 75 micron (3 mil). This can be tolerated with 200 micron fiber and a 300 micron projected spot size. The problem is the significant, over fifty percent, reduction in light flux launched into the fiber even under optimum conditions. Worse, for the manufacturer, the art appears to be standardizing on 85 micron fiber core. With a worst-case cumulative error of 75 micron, the light loss will be much higher with this fiber.
In view of the foregoing, it is therefore an object of the present invention to provide an improved assembly method for optoelectronic devices.
Another object of the present invention is to eliminate cumulative error in assembling optoelectronic devices.
A further object of the present invention is to provide low cost, high performance optoelectronic devices using standard piece parts.
Another object of the present invention is to provide improved optoelectronic devices in which the diameter of the projected spot size is less than the sum of the diameter of the optical fiber plus the tolerance of the header to the cap.
A further object of the present invention is to eliminate the need for highly accurate die bonding.
Another object of the present invention is to optimize coupling efficiency by enabling the diameter of the projected spot to be equal to the diameter of the core of the adjacent fiber.